Aiming to alleviate the stability problem for the parallel operation of multiple photovoltaic(PV)power generation units(MPGUs)connected to weak AC system

a small-signal model is developed with two parallel PV units. This model is used to study the influence of power grid strength

the operation states of each PV unit

and the control parameters in each phase-locked loop(PLL)controller on the stability of multiple PV units in parallel operation. Then the operation coupling mechanism of each PV unit is revealed. In addition

a control strategy for PV power generation is proposed based on the virtual terminal voltage

which can equivalently reduce the impedance of the power grid and improve the operation stability of multiple paralleled PV units connected to the weak AC system. The results show that with the increasing of output power of the PV array and the decreasing of power grid strength

the multiple PV units are prone to oscillation; the increasing of proportional coefficient of PLL controller in each inverter can enhance the system damping and improve the stability of the system in a certain range(the influence of integral coefficient is not considered); the power loop of the multiple PV units may have a coupling term in comparison with that of the single one connected to the weak AC system

which is susceptible to the operation state of PV units and the proportional coefficient of PLL controller

hence it is not conducive to the system stability. The effectiveness of theoretical analysis and the feasibility of the control strategy are verified based on the time-domain simulation examples.